Batten-roll and batten-cap metal roofing with lap-joints have been around for centuries. Similarly, patent glazing incorporating metal and glass has been used very successfully on buildings for many years but unjointed examples are restricted by the length of the glass panes. In contrast, standing seem continuous tray roofing is comparatively new. It is becoming increasingly common, however, because of its secret clip-fixing which does not penetrate the external coverings. The slope necessary to drain these designs has been reduced to a few degrees but there are a few examples of this technology which are true pedestrian areas. In order to ensure water tightness it has hitherto been necessary to incorporate continuous impervious membranes in roofs with shallow falls. This usually precludes the design from being vapor permeable without special detailing for this, and thermal expansion. Such membranes are usually materials which do not conduct thermal energy efficiently and are consequently poor candidates for collecting solar energy.